The need for secrecy in communications is an everlasting theme extending from ancient times to the future, and in the recent network society, the particular need has been ensured by the advancement of cryptography. The security of the currently widely used public key cryptosystem and the like depends on the fact that an unrealistic time is required for decoding. However, because computer technology is continuing to make constant progress, the above does not mean that the security of the public key cryptosystem and the like is guaranteed over the future. In contrast, quantum cryptography on which active researches are currently being done has its security guaranteed by the laws of physics, and no matter how far technology progresses, the security of quantum cryptography will not deteriorate. The realization thereof is being hoped for in this context.
The quantum cryptography, currently closest to practical use, is the quantum key distribution scheme using faint LD light, described in Non-Patent Document 1. This scheme utilizes the laws of quantum mechanics to share a necessary common key between a message sender and a recipient, and perform encrypted normal communications after the common key has been shared. During the process of sharing the common key, a random-number signal is transmitted using an exclusive optical line with the average photon number of less than one for one signal. Because one signal is constructed using less than one photon of light, even if this signal is eavesdropped on, the legitimate recipient can detect this fact and generate the common key by using only the random-number data whose successful receiving without being eavesdropped on has been ascertainable. Although the security of this scheme is already proved in cryptographic terms, the scheme always requires an exclusive line and is extremely weak against transmission loss because the number of photons used for one signal is less than one. For instance, 100-km transmission reduces the generating rate of the key to about several bits per second (bps). These drawbacks suggest that the introduction of the quantum key distribution scheme which uses faint LD light will be confined to limited use.
Under the background, Yuen et al. have proposed (in Non-Patent Document 2) a quantum-mechanical scheme that uses a mesoscopic number of photons to transmit the signal itself as well as to deliver a key (“mesoscopic” is a term that means somewhere in between “macroscopic” and “microscopic”). The two quadrature components (or paired intensity and phase) of light are not determined simultaneously below the accuracy of its quantum-mechanical fluctuation. Changing a transmission basis finely in a phase modulation scheme and ensuring that adjacent transmission bases are included in the range of quantum-mechanical fluctuation makes it impossible for eavesdroppers unknowing of these transmission bases to retrieve meaningful information from eavesdropping signals. It is reported in Non-Patent Document 3, however, that in this scheme, although the bases assuredly become uncertain within the range of quantum-mechanical fluctuation, if the pseudo-random numbers, which are used in ordinary encryption, are used during the process of changing the basis, when the photon number per signal is increased, the security of the scheme will be no more than that of ordinary classical mechanics-based encryption. In the present situation, the quantum-mechanical scheme mentioned above is still at its research phase.
Although the photon number in the method by Yuen et al. is limited to a mesoscopic number, this method has departed from using less than one photon of faint light and been invented in view not only of distributing a key, but also of sending the signal itself. Hence, the above method is an invention that has approached a realistic position. [Non-Patent Document 1] N. Gisin, G. Ribordy, W. Tittel, and H. Zbinden, Reviews of Modern Physics 74, 145-195 (2002).    [Non-Patent Document 2] G. A. Barbosa, E. Corndorf, P. Kumar, and H. Yuen, Physical Review Letters 90, No. 22, 227901 (2003).    [Non-Patent Document 3] T. Nishioka, T. Hasegawa, H. Ishizuka, K. Imafuku, and H. Imai, arXiv: quant-ph/0310168 v2 31 Oct. 2003 (http://xxx.lan1.gov/).